A measurement receiver is typically added to a transmitter for providing a measurement of the transmit signal power back to the transmitter which in turn uses the signal power measurement for transmit gain adjustments. Conventional measurement receivers rely mostly on the RMS (root-mean-square) power estimation principle. Using the RMS methodology, the transmitted RF signal is fed back to the measurement receiver via a coupler. The coupled transmit signal is then gain controlled by an automatic gain controller, demodulated, filtered, and converted by an ADC (analog-to-digital converter) to a digital waveform for RMS power estimation. The transmitter adjusts the transmit forward gain settings in response to the RMS power estimate to ensure that the transmit signal power complies with system requirements such as those mandated by 3GPP (3rd Generation Partnership Project), LTE (Long Term Evolution) or other standards or requirements. The measurement receiver also measures the phase shift introduced by the transmitter when the power amplifier changes gain states, e.g. from minimum to maximum gain. The transmitter uses the phase measurement to implement appropriate phase compensation.
Gain and phase estimates can be obtained by comparing the magnitude and phase of a copy of the transmitted signal relative to the magnitude and phase of a reference signal. The theoretical limit for gain estimation is one sample. In practice however, gain and phase estimation time depends on the amount of distortion in the transmitter power amplifier, noise in the transmitter and measurement receiver, and the time alignment achieved between the reference signal and the copied transmit signal. Reliable gain and phase estimates must be obtained in time periods much shorter than those needed to obtain reliable RMS power estimates.
RMS power estimation typically requires a long observation time, in many cases more than the duration of one time slot depending on the standard, modulation type, channel bandwidth, and configuration. For example, up to 450 μs may be needed to estimate the transmit power to within acceptable accuracy levels (e.g. 0.1 dB of error) for a single resource block of a QPSK (Quadrature Phase Shift Keying) LTE signal for PUCCH (Physical Uplink Control Channel) or PUSCH (Physical Uplink Shared Channel). On the other hand, transmit power estimation may take up to 10 ms for a single resource block of a QAM (Quadrature Amplitude Modulation) LTE signal. The estimation times in both cases are prohibitively long for the same accuracy and may extend well beyond the slot duration for LTE and WCDMA (Wideband CDMA). Transmit power control is normally conducted on a per-slot basis. As such, an accurate estimate of the transmit power is needed over a small fraction of the slot duration. Furthermore, shorter measurement times provide supply power savings in the transmitter platform.
In the case of WCDMA, a typical timing requirement for obtaining a meaningful power measurement is about 25 us from the beginning of a WCDMA slot boundary. This strict timing requirement places a burdensome constraint on using RMS power measurement techniques. Another draw back of conventional RMS power estimation techniques is that a phase measurement is not produced.
Gain and phase estimation on the other hand require adjusting the delay between the reference signal and the copied transmit signal on a per-slot basis to achieve the desired estimation accuracy. Delay estimation is required due to variations in process, temperature and loading effects, as well as delays that arise along the various signal paths, e.g. due to filtering, etc. Using a fixed delay value between the reference signal and copied transmit signal can significantly degrade gain and phase estimation accuracy. Due to strict timing requirements, a quick delay estimation methodology must be employed to yield the desired accuracy in the gain and phase estimates. Also, due to the presence of noise in the copy of the transmitted signal, a more efficient approach should be employed to estimate the phase accurately.